Field of the Invention
The present invention concerns a method for automated determination of the resonance frequencies for protons for magnetic resonance examinations, as well as a magnetic resonance apparatus that is designed to implement such a method.
Description of the Prior Art
The resonance frequency of a magnetically resonant nucleus is obtained from the so-called Larmor equation, which describes the dependence of the resonance frequency as a function of the field strength and the gyromagnetic ratio, a nucleus-specific variable. For protons, the resonance frequency lies at a field strength of 1.5 T at 63.5 MHz.
The phenomenon called chemical shift should be taken into account in addition. This describes deviations of the resonance frequency for spin species of a nucleus, this being dependent on the chemical environment of the nucleus. Thus, the resonance frequency of protons varies according to whether they are bound in fat, silicone or water. At 1.5 T, the difference between fat and water lies at 3.5 ppm, or 225 Hz.
The resonance frequency also varies for a single spin species, e.g. water protons, from examination to examination. This means that calibration measurements must be carried out when the examination subject changes or there is a change in the position of the examination subject, in order to enable an optimally automated data acquisition.
After the examination subject, a patient for example, has been introduced, the magnetic field can be homogenized. This process is also referred to as “shimming”. This entails adjusting the currents in components known as shim coils so that a maximally homogeneous magnetic field is produced and as a result the decay times T2 and T2* are maximized. This process can be automated.
Furthermore, the resonance frequency can also be determined automatically. To that end, an FID is recorded and Fourier-transformed, after which it is present as a spectrum. This spectrum consists of individual elements in the form of numeric values, which accordingly represent a vector.
In this spectrum, each spin species essentially has one peak. The spin species attributed to the fat category due to the chemical factors in fact possess a number of peaks, but of these one is dominant. When reference is made in the following to “a” fat peak or resonance signal, this does not preclude the presence of further peaks. It simply means that only one peak is relevant insofar as the method according to the invention is concerned.
In order to record the FID, a frequency that corresponds e.g. to the last-used resonance frequency of the corresponding nucleus is used as the resonance frequency. The resonance frequency corresponding to the resonance frequency of the nucleus or of its main component may also be used. In the case of protons, this is the water resonance.
The spectrum can have different resonance peaks depending on the examination subject and the examination region. Peaks may be present for water alone, for water and fat, or for water and silicone, or also for all three nucleus species.
In order to determine the correct resonance frequency or correct resonance frequencies from the measured spectrum, it is furthermore known to have recourse to model spectra having one or two peaks with different signal intensities and also varying distances between the peaks, in order to determine cross-correlation coefficients by means of the measured spectrum. The more model spectra that are used, the better will be the result of the cross-correlation analysis.
The model spectra are also present in vector form, which means that the calculation of the cross-correlation coefficients is a vector operation that is repeated multiple times.
This approach has the disadvantage that it is intensive in terms of computing time and consequently it is necessary for a compromise between the time needing to be invested and the requisite quality.